qt: HACKING: update virtual vs override

Now that Q_DECL_OVERRIDE is gone, update HACKING for the usage of
override.

modules/gui/qt/HACKING

@@ -43,13 +43,13 @@ So if your dialog is a dialog you are used to keep during a playing of a video,
 QVLCFrame: extended panel, messages...
 QVLCDialog: preferences, open...
 
-** virtual vs Q_DECL_OVERRIDE **
+** virtual vs override **
 Only mark functions with the virtual keyword if this is the initial virtual
 declaration. Functions that override a virtual function should have the
-Q_DECL_OVERRIDE macro appended (which expands to the override identifier
-when compiled with Qt5 and C++11). This is a visual indicator to distinguish
-virtual declarations from overriding implementations and will include a
-compile-time check in C++11 language mode.
+override identifier appended. Not only is this a visual indicator to distinguish
+virtual declarations from overriding implementations, it will also include a
+compile-time check to ensure that the function actually overrides another
+virtual function, with matching name and signature.
 
 ** HIG for Qt **
 We tend to use GNOME/KDE specs over Vista ones for capitalizations: